package com.edu.policy.utils;

public class SnowflakeIdGenerator {
    // 起始的时间戳，这个时间戳可以设置为项目开始运行的时间
    private final static long START_TIMESTAMP = 1713771118000L; // 2024-04-22 15:31:18
    // 每一部分占用的位数
    private final static long SEQUENCE_BIT = 12; // 序列号占用的位数
    private final static long MACHINE_BIT = 5;   // 机器标识占用的位数
    private final static long DATACENTER_BIT = 5;// 数据中心占用的位数
    // 每一部分的最大值
    private final static long MAX_SEQUENCE = ~(-1L << SEQUENCE_BIT);
    private final static long MAX_MACHINE_NUM = ~(-1L << MACHINE_BIT);
    private final static long MAX_DATACENTER_NUM = ~(-1L << DATACENTER_BIT);
    // 每一部分向左的位移
    private final static long MACHINE_LEFT = SEQUENCE_BIT;
    private final static long DATACENTER_LEFT = SEQUENCE_BIT + MACHINE_BIT;
    private final static long TIMESTAMP_LEFT = DATACENTER_LEFT + DATACENTER_BIT;

    private final long datacenterId;  // 数据中心
    private final long machineId;     // 机器标识
    private long sequence = 0L; // 序列号
    private long lastTimestamp = -1L; // 上一次时间戳

    public SnowflakeIdGenerator(long datacenterId, long machineId) {
        if (datacenterId > MAX_DATACENTER_NUM || datacenterId < 0) {
            throw new IllegalArgumentException("DatacenterId can't be greater than " + MAX_DATACENTER_NUM + " or less than 0");
        }
        if (machineId > MAX_MACHINE_NUM || machineId < 0) {
            throw new IllegalArgumentException("MachineId can't be greater than " + MAX_MACHINE_NUM + " or less than 0");
        }
        this.datacenterId = datacenterId;
        this.machineId = machineId;
    }

    // 生成下一个ID
    public synchronized long nextId() {
        long timestamp = timeGen();
        if (timestamp < lastTimestamp) {
            throw new RuntimeException("Clock moved backwards.  Refusing to generate id");
        }

        if (lastTimestamp == timestamp) {
            sequence = (sequence + 1) & MAX_SEQUENCE;
            if (sequence == 0L) {
                timestamp = tilNextMillis(lastTimestamp);
            }
        } else {
            sequence = 0L;
        }

        lastTimestamp = timestamp;

        return ((timestamp - START_TIMESTAMP) << TIMESTAMP_LEFT) // 时间戳部分
                | (datacenterId << DATACENTER_LEFT) // 数据中心部分
                | (machineId << MACHINE_LEFT) // 机器标识部分
                | sequence; // 序列号部分
    }

    // 获取当前时间戳
    private long timeGen() {
        return System.currentTimeMillis();
    }

    // 获取下一个时间戳
    private long tilNextMillis(long lastTimestamp) {
        long timestamp = timeGen();
        while (timestamp <= lastTimestamp) {
            timestamp = timeGen();
        }
        return timestamp;
    }
}
